This invention relates to an improved process for removing unexposed diazo material from an aluminum lithographic printing plate.
Lithographic printing techniques, using, for example, silicated aluminum plates as described in Jewett et al U.S. Pat. No. 2,714,066, or anodized and silicated aluminum plates as described in Fromson U.S. Pat. No. 3,181,461, are widely used in the printing industry.
A negative working plate is coated with an aqueous solution of a diazo resin, dried and normally exposed through a negative for 1 to 2 minutes. The exposed image areas become water insoluble and the unexposed non-image areas remain water soluble. The plate is conventionally developed with a lithographic lacquer which consists of a two-phase system, one phase containing an hydrophilic gum in water and the other an oleophilic resin in a solvent. Upon application, the oleophilic resin adheres to the exposed insoluble areas, while the water in the aqueous phase dissolves away the unexposed soluble non-image areas leaving a deposit of gum in the same areas. In this way, the image areas are made oleophilic or ink receptive and the gummed, non-image areas are made hydrophilic or ink repellent.
Silicated aluminum plates and silicated anodized aluminum plates (which can be pretreated to produce a grained surface) have been found to have a negatively charged surface which forms an ionic bond with a diazo resin which has an opposite, positive charge. While the unexposed, non-image portions of the diazo in theory remain water soluble, in practice it has been found that the dissolving power of water is insufficient to overcome the ionic bond. Thus, residual diazo remains in the non-image areas and the art has employed gum arabic to mask the residual diazo, which remains olephilic, to prevent scumming or ink pick-up in the non-image areas (also referred to as the background).
The presence of residual diazo in the background can be demonstrated by exposing a diazo coated silicated aluminum plate thru a negative, developing it with water and then re-exposing the plate thru the same negative turned ninety degrees. The twice exposed plate is again developed with water and then rubbed with press ink. The first image appears as well as a second, crossed image in the background of the first image. Ionically bonded diazo that remains in the background is responsible and, unless masked with gum arabic, will result in undesirable ink pick-up in the background long before the image wears.
The use of stronger solvents such as benzyl alcohol with water or by themselves has been proposed as well as coupling diazo resins with various materials to make them sparingly soluble in water and soluble in organic solvents (cf. U.S. Pat. Nos. 3,300,309 and 3,790,556). However, solvents are expensive and present serious disposal problems. The art has long sought a truly water-developable plate.
It has also been proposed to alter the surface characteristics of the aluminum plate to minimize or prevent ionic bonding by the diazo resin (cf U.S. Pat. No. 3,220,832) but this has drawbacks with respect to adhesion in the image areas.
While there are suggestions for using wetting agents in lithographic developers and lacquers, the art has avoided the use of anionic surfactants in diazo developers because it is known that a chemical reaction takes place (cf. U.S. Pat. No. 3,790,556) between a diazo and an anionic surfactant. The ink-loving reaction product is difficult to completely remove and unless it is masked with a water-loving substance such as gum arabic, its presence in the background will cause undesirable scumming or ink pick-up.
The present invention provides a developing system using an aqueous developer that removes the soluble light sensitive material completely. The invention also overcomes ecological problems by providing an aqueous system which can replace heretofore used organic solvent systems.